Manufacturing method of composite oxide and manufacturing method of power storage device

ABSTRACT

An object is to reduce variation in shape of crystals that are to be formed. Solutions containing respective raw materials are made in an environment where an oxygen concentration is lower than that in air, the solutions containing the respective raw materials are mixed in an environment where an oxygen concentration is lower than that in air to form a mixture solution, and with use of the mixture solution, a composite oxide is formed by a hydrothermal method.

TECHNICAL FIELD

The present invention relates to a composite oxide and further relatesto a power storage device.

BACKGROUND ART

In recent years, power storage devices such as lithium ion secondarybatteries have been developed.

Examples of such power storage devices include a power storage devicehaving an electrode formed using lithium iron phosphate (LiFePO₄), whichis a composite oxide, as an active material. The power storage devicehaving an electrode formed using LiFePO₄ has high thermal stability andfavorable cycle characteristics.

As an example of a method for forming a composite oxide such as LiFePO₄,a hydrothermal method can be used (e.g., Patent Document 1). Ahydrothermal method is a method, which is performed in the presence ofhot water, for synthesizing a compound or growing crystal of a compound.

By using a hydrothermal method, even a material which is less likely tobe dissolved in water at ordinary temperatures and pressures can bedissolved, and thus a substance which is hardly obtained by a productionmethod performed at ordinary temperatures and pressures can besynthesized or crystal growth of such a substance can be conducted.Further, by using a hydrothermal method, microparticles of singlecrystals of an objective substance can be easily synthesized.

Using a hydrothermal method, for example, enables a desired compound tobe formed in the following manner: a solution containing a raw materialis introduced into a container resistant to pressure and be subjected toheat and pressure treatment; and the treated solution is filtered.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2004-095385

DISCLOSURE OF INVENTION

However, LiFePO₄ formed by the conventional hydrothermal method has aproblem of large variation in crystal shape of LiFePO₄.

When variation in crystal shape is large, for example, the bulk densityof crystals in an active material of an electrode is low, and thecharge-discharge characteristics of a power storage device are reduced.In order to suppress a reduction in the charge-discharge characteristicsof the power storage device, crystal shapes are preferably regulated, sothat the crystal shapes in the active material of the electrode areuniform.

An object of one embodiment of the present invention is to reducevariation in shape of crystals formed by using a hydrothermal method.

In one embodiment of the present invention, a mixture solutioncontaining a raw material is made in an environment where an oxygenconcentration is lower than that in air, and with use of the mixturesolution, a composite oxide is formed by using a hydrothermal method,whereby difference in crystal shape in the formed composite oxide issuppressed.

One embodiment of the present invention is a production method of acomposite oxide including the steps of: forming a solution containinglithium (Li) with use of a compound containing Li in an atmosphere wherean oxygen concentration is lower than that in air; forming a solutioncontaining phosphorus (P) with use of a compound containing P in anatmosphere where an oxygen concentration is lower than that in air;forming a solution containing one or more transition metals selectedfrom iron (Fe), cobalt (Co), nickel (Ni), and manganese (Mn) with use ofone or more compounds containing any one transition metal of Fe, Co, Ni,and Mn in an atmosphere where an oxygen concentration is lower than thatin air; mixing the solution containing Li and the solution containing Pin an atmosphere where an oxygen concentration is lower than that in airand forming a solution containing Li and P; mixing the solutioncontaining Li and P and the solution containing the transition metal inan atmosphere where an oxygen concentration is lower than that in airand forming a mixture solution; and forming a composite oxide by ahydrothermal method in an atmosphere where an oxygen concentration islower than that in air. The thus obtained composite oxide is representedby a general formula, LiMPO₄ (M denotes one or more of Fe, Co, Ni, andMn).

According to one embodiment of the present invention, shapes of formedcrystals can be regulated. Thus, difference in crystal shape can bereduced.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are flow charts showing an example of a productionmethod of a composite oxide.

FIG. 2 is a schematic view illustrating an example of a crystalstructure of a composite oxide.

FIG. 3 illustrates an example of a power storage device.

FIGS. 4A and 4B each illustrate a structure example of an electrode in apower storage device.

FIG. 5 illustrates an example of a power storage device.

FIG. 6 illustrates examples of electric devices.

FIG. 7A is a schematic view illustrating an example of an electricdevice and FIG. 7B is a block diagram thereof.

FIG. 8 shows an observation result of LiFePO₄, which is obtained with ascanning electron microscope.

FIG. 9 shows an observation result of LiFePO₄, which is a comparisonexample and obtained with a scanning electron microscope.

BEST MODE FOR CARRYING OUT THE INVENTION

Examples of embodiments of the present invention will be described withreference to the drawings below. Note that it will be readilyappreciated by those skilled in the art that details of the embodimentscan be modified in various ways without departing from the spirit andscope of the invention. Thus, the present invention should not belimited to the description of the following embodiments.

Note that the contents in different embodiments can be combined with oneanother as appropriate. In addition, the contents of the embodiments canbe replaced with each other as appropriate.

Further, the ordinal numbers such as “first” and “second” are used toavoid confusion between components and do not limit the number of eachcomponent.

(Embodiment 1)

In this embodiment, an example of a composite oxide having crystals willbe described.

An example of a production method of a composite oxide of thisembodiment is described with reference to flow charts in FIGS. 1A and1B.

In the example of a production method of a composite oxide in thisembodiment, as shown in FIG. 1A, a mixture solution used forsynthesizing a composite oxide is formed in an atmosphere where anoxygen concentration is lower than that in air (the atmosphere is alsoreferred to as “low-oxygen concentration atmosphere”) as a step S11.Then, a composite oxide is formed with use of the obtained mixturesolution by a hydrothermal method as a step S12.

Details of the production method of a composite oxide is described witha flow chart shown in FIG. 1B.

In the example of a production method of a composite oxide of thisembodiment, as shown in FIG. 1B, a compound containing lithium (Li),which is also referred to as “lithium compound”, is weighed as a stepS11_1. Further, a compound containing phosphorus (P), which is alsoreferred to as “phosphorus compound”, is weighed as a step S11_2.Further, a compound containing a transition metal, which is alsoreferred to as “transition metal compound”, is weighed as a step S11_3.

Examples of lithium compounds that can be used include lithium hydroxidemonohydrate (LiOH.H₂O), anhydrous lithium hydroxide (LiOH), lithiumcarbonate (Li₂CO₃), lithium oxide (Li₂O), lithium nitrate (LiNO₃), andlithium acetate (CH₃COOLi).

Examples of phosphorus compounds that can be used include phosphoricacid such as orthophosphoric acid (H₃PO₄) and ammoniumhydrogenphosphates such as ammonium dihydrogenphosphate (NH₄H₂PO₄).

As a transition metal compound, for example, one or more compoundscontaining any one of iron (Fe), cobalt (Co), nickel (Ni), and manganese(Mn) can be used. Examples thereof include iron chloride tetrahydrate(FeCl₂.4H₂O), iron sulfate heptahydrate (FeSO₄.7H₂O), iron acetate(Fe(CH₃COO)₂), manganese chloride tetrahydrate (MnCl₂.4H₂O), manganesesulfate-hydrate (MnSO₄.H₂O), manganese acetate tetrahydrate(Mn(CH₃COO)₂.4H₂O), cobalt chloride hexahydrate (CoCl₂.6H₂O), cobaltsulfate (CoSO₄), cobalt acetate tetrahydrate (Co(CH₃COO)₂.4H₂O), nickelchloride hexahydrate (NiCl₂.6H₂O), nickel sulfate hexahydrate(NiSO₄.6H₂O), and nickel acetate tetrahydrate (Ni(CH₃COO)₂.4H₂O).

Then, as a step S12_1, the lithium compound is dissolved in a solvent inan atmosphere where an oxygen concentration is lower than that in air,so that a solution containing Li (also referred to as “lithiumcontaining solution”) is formed. Further, as a step S12_2, thephosphorus compound is dissolved in a solvent in an atmosphere where anoxygen concentration is lower than that in air, so that a solutioncontaining P (also referred to as “phosphorus containing solution”) isformed. Further, as a step S12_3, the transition metal compound isdissolved in a solvent in an atmosphere where an oxygen concentration islower than that in air, so that a solution containing a transition metal(also referred to as “transition metal-containing solution”) is formed.

As the solvent in which the lithium compound, the phosphorus compound,or the transition metal compound is dissolved, water can be used forexample. In addition, the oxygen concentration in the solvent ispreferably 4.5 ppm or lower. For example, nitrogen bubbling is performedin the solvent, whereby the oxygen concentration in the solvent can bereduced. Reduction in the oxygen concentration in the solvent enablessuppression of oxidation of a substance which is to be produced.

Further, as the atmosphere where an oxygen concentration is lower thanthat in air, for example, a nitrogen atmosphere or a mixed atmosphere ofnitrogen and hydrogen can be employed.

Next, as a step S13, the solution containing Li and the solutioncontaining P are mixed in an atmosphere where an oxygen concentration islower than that in air, so that a solution containing Li and P (alsoreferred to as “lithium-phosphorus-containing solution”) is formed. Thesolution containing Li and P is slightly alkaline.

Then, as a step S14, the solution containing Li and P and the solutioncontaining the transition metal are mixed in an atmosphere where anoxygen concentration is lower than that in air, so that a mixturesolution is formed.

In this step, it is preferable, for example, to drip the solutioncontaining Li and P little by little while stirring the solutioncontaining the transition metal. Thus, a hydrogen ion in the solutioncontaining the transition metal and a hydroxide ion in the solutioncontaining Li and P are reacted (neutralization reaction) prior to areaction of a transition metal ion and a hydroxide ion. As a result,production of unnecessary transition metal hydroxide can be reduced.

Then, as a step S15, a composite oxide is formed using the mixturesolution by a hydrothermal method.

For example, the mixture solution is introduced into a containerresistant to pressure, subjected to pressure and heat treatment, andcooled. Then, the cooled solution is filtered. In this manner, thecomposite oxide can be formed.

As a container resistant to pressure, an autoclave or the like can beused, for example.

A preferable temperature at which the pressure and heat treatment isperformed is, for example, higher than or equal to 100° C. and lowerthan or equal to the critical temperature of water. A preferablepressure at which the pressure and heat treatment is performed is, forexample, higher than or equal to 0.1 MPa and lower than or equal to thecritical pressure of water. A preferable time during which the pressureand heat treatment is performed is, for example, 0.5 hours or longer. Inaddition, the atmosphere in the container resistant to pressure is setto have lower oxygen concentration than that in air, whereby thetreatment can be performed by a hydrothermal method in an atmospherewhere an oxygen concentration is lower than that in air. For example,the preferable atmosphere in the container resistant to pressure is anitrogen atmosphere or a mixed atmosphere of nitrogen and hydrogen.Thus, unnecessary oxygen in the container resistant to pressure can beremoved. Further, even when the solution is oxidized during the step,the solution can be reduced. Therefore, adverse effect of oxidation canbe suppressed.

Alternatively, the pressure and heat treatment may be performed afteraddition of a reducing agent. For example, a reducing agent is added tothe mixture solution, and then the pressure and heat treatment can beperformed. Further, without limitation thereto, for example, a reducingagent may be added to the solution containing the transition metal, andthen the mixture solution may be formed.

Examples of a reducing agent include ascorbic acid, sulfur dioxide,sulfurous acid, sodium sulfite, sodium hydrogen sulfite, ammoniumsulfite, and phosphorous acid. With use of a reducing agent, thesolution can be reduced even when the solution is oxidized during thestep; thus, adverse effect of oxidation can be reduced.

The composite oxide obtained by a hydrothermal method has a plurality ofcrystals. The crystal shape of the composite oxide is, for example, arectangular solid 161 as illustrated in FIG. 2. Variation in shapes ofthe plurality of crystals, which are rectangular solids, is small. Thisis because impurities in the composite oxide formed under the conditionwhere oxidation is suppressed are reduced. Note that it is preferablethat a crystal structure of the composite oxide be an olivine structure.

The foregoing has described an example of a production method of acomposite oxide according to this embodiment.

As described with reference to FIGS. 1A and 1B and FIG. 2, according tothe example of a production method of a composite oxide of thisembodiment, a solution containing lithium (Li) and phosphorus (P) isformed by mixing a solution containing Li and a solution containing P inan atmosphere where an oxygen concentration is lower than that in air,and then a mixture solution is formed by mixing the solution containingLi and P and a solution containing a transition metal in an atmospherewhere an oxygen concentration is lower than that in air, so thatgeneration of by-product due to oxidation can be suppressed. Thus,variation in crystal shape of composite oxide formed by using ahydrothermal method can be small.

(Embodiment 2)

In this embodiment, an example of a power storage device having anelectrode in which the composite oxide described in Embodiment 1 is usedfor an active material will be described.

A structure example of a power storage device in this embodiment isdescribed with reference to FIG. 3.

The power storage device illustrated in FIG. 3 includes a positiveelectrode 201, a negative electrode 202, an electrolyte 203, and aseparator 204.

The positive electrode 201 includes a positive electrode currentcollector 211 and a positive electrode active material layer 212.

For the positive electrode current collector 211, aluminum, copper,nickel, titanium, or the like can be used. Further, an alloy including aplurality of materials capable of being used for the positive electrodecurrent collector 211 may be used.

For the positive electrode active material layer 212, for example, thecomposite oxide described in Embodiment 1 can be used. In this case, thecomposite oxide can be formed, for example, by the production methoddescribed in Embodiment 1. The composite oxide functions as an activematerial.

For example, a conduction auxiliary agent, a binder, and a solvent areadded to the composite oxide described in Embodiment 1, whereby a pasteis formed. The paste is applied on the positive electrode currentcollector 211 and baked, so that the positive electrode active materiallayer 212 can be formed.

The negative electrode 202 includes a negative electrode currentcollector 221 and a negative electrode active material layer 222.

For the negative electrode current collector 221, iron, copper, nickel,or the like can be used. Alternatively, for the negative electrodecurrent collector 221, an alloy of a plurality of materials capable ofbeing used for the negative electrode current collector 221 may be used.

For the negative electrode active material layer 222, silicon, graphite,or the like can be used. In this case, silicon or graphite functions asan active material.

Further, the negative electrode active material layer 222 may have astructure including a plurality of whiskers, for example.

Further, for the negative electrode active material layer 222, graphenecan be used for example.

Graphene refers to a one-atom-thick sheet of carbon molecules having sp²bonds and holes through which ions pass, or refers to a stack in which aplurality of one-atom-thick sheets (the number of sheets is 2 to 100)are stacked (the stack is also referred to as multilayer graphene).Further, net-like graphene is referred to as graphene net. Note that ingraphene, an element other than carbon may be contained to account for30 at. % or less, or an element other than carbon and hydrogen may becontained to account for 15 at. % or less. Thus, graphene analogue isalso regarded as graphene.

Characteristics of graphene are high conductivity, sufficientflexibility, high mechanical strength and high heat resistance. Inaddition, graphene has capacity of storing ions.

In the case where silicon or graphite is used for an active material,for example, silicon or graphite may be covered with graphene. Further,in the case of multilayer graphene, for example, microparticles ofsilicon, graphite, or the like may be included between a plurality ofgraphene layers.

By using graphene, conductivity of an electrode can be increased.Accordingly, graphene can have a function of a binder. Further, by usinggraphene, an electrode can be formed without a conventional conductiveauxiliary agent or a binder.

Furthermore, by using graphene, deformation and damage of the electrodecan be suppressed.

Note that graphene may be used not only for the negative electrodeactive material layer 222 but also for the positive electrode activematerial layer 212. For example, a plurality of crystals of thecomposite oxide described in Embodiment 1 may be covered with graphene.Further, in the case of multilayer graphene, for example, microparticlesof the composite oxide may be provided between a plurality of graphenelayers.

FIG. 4A illustrates a structure example of the negative electrode 202including the negative electrode active material layer 222. In thisexample, the negative electrode active material layer 222 has aplurality of whiskers.

The electrode illustrated in FIG. 4A includes the negative electrodecurrent collector 221 and the negative electrode active material layer222. The negative electrode active material layer 222 includes a layer251 having whiskers.

As the layer 251 having whiskers, for example, silicon can be used.

In the layer 251 having whiskers, it is preferable that a core portionof each whiskers have crystallinity (the core portion is also referredto as crystalline portion) and the peripheral portion of the coreportion be amorphous. For example, in the amorphous portion, change involume, which occurs due to store and release of ions, is small. Thecrystalline portion has high conductivity and high ion conductivity;thus, in the crystalline portion, the rate of storing ions and the rateof releasing ions can be increased.

The layer 251 having whiskers can be formed by, for example, a lowpressure CVD (LPCVD) method.

For example, in the case where the layer 251 having whiskers is formedusing silicon, a deposition gas containing silicon can be used as asource gas for forming the layer 251 having whiskers by an LPCVD method.For the deposition gas containing silicon, silicon hydride, siliconfluoride, or silicon chloride can be used for example.

The pressure is preferably set to 10 Pa to 1000 Pa inclusive, furtherpreferably 20 Pa to 200 Pa inclusive. By adjusting pressure, thecrystalline portion and the amorphous portion can be each formed.

Alternatively, the negative electrode active material layer 222 can havesuch a structure that a layer having graphene is provided to cover thelayer 251 having whiskers. FIG. 4B illustrates a structure example ofthe negative electrode 202 including the negative electrode activematerial layer 222 in which a layer having graphene is provided to coverthe layer having whiskers.

The electrode illustrated in FIG. 4B includes a layer 252 havinggraphene in addition to the structure illustrated in FIG. 4A.

The layer 252 having graphene is provided in contact with the layer 251having whiskers.

For example, the layer 252 having graphene can be formed in thefollowing manner: a graphene oxide layer is formed over the layer 251having whiskers and then heated, so that the graphene oxide layer isreduced.

As an example illustrated in FIG. 4B, the negative electrode activematerial layer has a structure in which a layer having graphene isprovided in contact with the layer having whiskers, whereby even whenthe volume of the layer having whiskers is changed due to store andrelease of ions, for example, the layer having graphene contributes toreduce stress caused by change in volume. As a result, the whiskerstructure in the layer having whiskers can be prevented from beingdamaged. Therefore, cycle characteristics of the power storage devicecan be improved.

As the separator 204 illustrated in FIG. 3, for example, paper, nonwovenfabric, glass fiber, or synthetic fiber may be used. As the syntheticfiber, materials such as nylon (polyimide), vinylon (also calledvinalon) (polyvinyl alcohol fiber), polyester, acrylic, polyolefin, andpolyurethane may be used. More examples of materials of the separator204 are polymer materials (high-molecular compounds) such asfluorine-based polymer, polyether (e.g., polyethylene oxide andpolypropylene oxide), polyolefin (e.g., polyethylene and polypropylene),polyacrylonitrile, polyvinylidene chloride, polymethyl methacrylate,polymethylacrylate, polyvinyl alcohol, polymethacrylonitrile, polyvinylacetate, polyvinylpyrrolidone, polyethyleneimine, polybutadiene,polystyrene, polyisoprene, and polyurethane, derivatives thereof, acellulose based material such as paper and film, and nonwoven fabric.These materials can be used either alone or in combination. However, itis necessary to choose a material which will not dissolve in theelectrolyte 203, as the separator 204.

The electrolyte 203 can be formed using a material including ionsserving as carriers, a material through which ions serving as carrierstransfer, or the like. Examples of such a material include lithiumchloride (LiCl), lithium fluoride (LiF), lithium perchlorate (LiClO₄),and lithium tetrafluoroborate (LiBF₄). These materials can be usedeither alone or in combination in the electrolyte 203. Alternatively,for the electrolyte 203, a lithium salt material containing fluorine canbe used, e.g., lithium hexafluorophosphate (LiPF₆), lithiumhexafluoroarsenate (LiAsF₆), lithium trifluoromethansulfonate(LiCF₃SO₃), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO₂CF₃)₂),lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO₂C₂F₅)₂), or thelike.

Alternatively, the electrolyte 203 can be formed by dissolving the abovematerial into a solvent. Examples of the solvent include a cycliccarbonate such as ethylene carbonate (EC), propylene carbonate (PC),butylene carbonate (BC), and vinylene carbonate (VC); an acycliccarbonate such as dimethyl carbonate (DMC), diethyl carbonate (DEC),ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC), isobutylmethyl carbonate, and dipropyl carbonate (DPC); an aliphatic carboxylicacid ester such as methyl formate, methyl acetate, methyl propionate,and ethyl propionate; a γ-lactone such as γ-butyrolactone; an acyclicether such as 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), andethoxymethoxy ethane (EME); a cyclic ether such as tetrahydrofuran and2-methyltetrahydrofuran; an alkyl phosphate ester such asdimethylsulfoxide, 1,3-dioxolane, trimethyl phosphate, triethylphosphate, and trioctyl phosphate; and fluorides thereof. Thesematerials can be used either alone or in combination as the solvent ofthe electrolyte 203.

As an example of a power storage device according to this embodiment, astructure example of a coin-type secondary battery is described withreference to FIG. 5.

The power storage device illustrated in FIG. 5 includes a positiveelectrode 301, a negative electrode 302, a separator 304, a housing 305,a housing 306, a ring-shaped insulator 307, a spacer 308, and a washer309.

The positive electrode 301 corresponds to the positive electrode 201 inFIG. 3, for example. A positive electrode current collector 311corresponds to the positive electrode current collector 211, and apositive electrode active material layer 312 corresponds to the positiveelectrode active material layer 212.

The negative electrode 302 corresponds to the negative electrode 202 inFIG. 3, for example. A negative electrode current collector 321corresponds to the negative electrode current collector 221, and anegative electrode active material layer 322 corresponds to the negativeelectrode active material layer 222.

The separator 304 corresponds to the separator 204 in FIG. 3, forexample.

The housing 305, the housing 306, the spacer 308, and the washer 309each of which is made of metal (e.g., stainless steel) are preferablyused. The housing 305 and the housing 306 have a function ofelectrically connecting the positive electrode 301 and the negativeelectrode 302 to the outside.

As in the case of the power storage device illustrated in FIG. 5, thepositive electrode 301, the negative electrode 302, and the separator304 are soaked in the electrolyte; the negative electrode 302, theseparator 304, the ring-shaped insulator 307, the positive electrode301, the spacer 308, the washer 309, and the housing 305 are stacked inthis order with the housing 306 positioned at the bottom; and thehousing 305 and the housing 306 are subjected to pressure bonding. Insuch a manner, the coin-type secondary battery is manufactured.

The foregoing has described an example of a power storage deviceaccording to this embodiment.

Note that not being limited to use for the above coin-type secondarybattery, the composite oxide according to Embodiment 1 can be used for arectangular or cylindrical secondary battery, for example.

As described with reference to FIG. 3, FIGS. 4A and 4B, and FIG. 5, inthe example of a power storage device according to this embodiment, thepositive electrode of the power storage device is formed using thecomposite oxide according to the above embodiment, whereby the bulkdensity of an active material can be increased. As a result, the energydensity of the power storage device can be increased.

(Embodiment 3)

In this embodiment, examples of electric devices each provided with apower storage device will be described.

Examples of electric devices using the power storage device are asfollows: display devices, lighting devices, desktop personal computersor notebook personal computers, image reproduction devices (for example,which reproduce a still image or a moving image stored in a storagemedium such as a digital versatile disc (DVD)), mobile phones, portablegame machines, portable information terminals, e-book readers, videocameras, digital still cameras, high-frequency heating apparatus such asmicrowaves, electric rice cookers, electric washing machines,air-conditioning systems such as air conditioners, electricrefrigerators, electric freezers, electric refrigerator-freezers,freezers for preserving DNA, dialysis devices, and the like. Inaddition, moving objects driven by an electric motor using electricpower from a power storage device are also included in the category ofelectric devices. As examples of the moving objects, electric vehicles,hybrid vehicles which include both an internal-combustion engine and anelectric motor, motorized bicycles including motor-assisted bicycles,and the like can be given.

Examples of electric devices of this embodiment are described withreference to FIG. 6 and FIGS. 7A and 7B.

A display device 5000 shown in FIG. 6 includes a housing 5001, a displayportion 5002, a speaker portion 5003, a power storage device 5004, andthe like. The display device 5000 corresponds to a display device for TVbroadcast reception.

Note that the display device disclosed in this specification includes,in its category, all of information display devices for personalcomputers, advertisement displays, and the like besides TV broadcastreception.

As the display portion 5002, a display device such as a liquid crystaldisplay device, a light-emitting device in which a light-emittingelement such as an organic EL element is provided in each pixel, anelectrophoresis display device, a digital micromirror device (DMD), aplasma display panel (PDP), or a field emission display (FED) can beused, for example.

The power storage device 5004 is provided in the housing 5001. As thepower storage device 5004, the power storage device described inEmbodiment 2 can be used for example.

The display device 5000 can receive electric power from a commercialpower supply. Alternatively, the display device 5000 can use electricpower stored in the power storage device 5004. Thus, even when electricpower cannot be supplied from the commercial power supply because ofpower outage or the like, the display device 5000 can be operated withuse of the power storage device 5004 as a power supply.

A lighting device 5100 shown in FIG. 6 is an installation lightingdevice. The lighting device 5100 includes a housing 5101, a light source5102, and a power storage device 5103.

As the light source 5102, an artificial light source which emits lightartificially by using electric power can be used. Examples of artificiallight source include discharge lamps such as an incandescent lamp and afluorescent lamp, and light-emitting elements such as a light-emittingdiode and an organic EL element.

The power storage device 5103 is provided in a ceiling 5104 on which thehousing 5101 and the light source 5102 are installed. Note that thepower storage device 5103 may be provided in the housing 5101, not beinglimited to the above case.

The lighting device 5100 can receive power from a commercial powersupply. Alternatively, the lighting device 5100 can use power stored inthe power storage device 5103. Thus, even when electric power cannot besupplied from the commercial power supply because of power outage or thelike, the lighting device 5100 can be operated with use of the powerstorage device 5103 as a power supply.

Note that although the installation lighting device 5100 shown in FIG. 6is provided in the ceiling 5104, it is not limited thereto. The powerstorage device can be used in an installation lighting device providedin a wall 5105, a floor 5106, a window 5107, or the like, besides in theceiling 5104. Alternatively, the power storage device can be used in atabletop lighting device and the like.

An air conditioner in FIG. 6 includes an indoor unit 5200 and an outdoorunit 5204.

The indoor unit 5200 includes a housing 5201, a ventilation duct 5202,the power storage device 5203, and the like. Note that although thepower storage device 5203 shown in FIG. 6 is provided in the indoor unit5200, it is not limited thereto. For example, the power storage device5203 may be provided in the outdoor unit 5204. Alternatively, the powerstorage devices 5203 may be provided in both the indoor unit 5200 andthe outdoor unit 5204.

The air conditioner can receive electric power from the commercial powersupply. Alternatively, the air conditioner can use electric power storedin the power storage device 5203. Specifically, in the case where thepower storage devices 5203 are provided in both the indoor unit 5200 andthe outdoor unit 5204, the air conditioner can be operated with use ofthe power storage device 5203 as a power supply even when electric powercannot be supplied from the commercial power supply because of poweroutage or the like.

Note that although the separated air conditioner including the indoorunit and the outdoor unit is shown in FIG. 6 as an example, the powerstorage device may be used in an air conditioner in which the functionsof an indoor unit and an outdoor unit are integrated in one housing.

An electric refrigerator-freezer 5300 shown in FIG. 6 includes a housing5301, a door for a refrigerator 5302, a door for a freezer 5303, and thepower storage device 5304.

The power storage device 5304 is provided in the housing 5301.

The electric refrigerator-freezer 5300 can receive power from acommercial power supply. Alternatively, the electricrefrigerator-freezer 5300 can use power stored in the power storagedevice 5304. Thus, even when electric power cannot be supplied from thecommercial power supply because of power outage or the like, theelectric refrigerator-freezer 5300 can be operated with use of the powerstorage device 5304 as a power supply.

In addition, a high-frequency heating apparatus such as a microwave andan electric device such as an electric rice cooker require high power ina short time. The stop of supplying a commercial power supply in use ofelectric devices can be prevented by using the power storage device asan auxiliary power supply for supplying electric power which cannot besupplied enough by a commercial power supply.

In addition, in a time period when electric devices are not used,specifically when the proportion of the amount of power which isactually used to the total amount of power which can be supplied by acommercial power supply source (such a proportion referred to as usagerate of power) is low, power can be stored in the power storage device,whereby an increase in usage rate of power can be suppressed in a timeperiod when the electric devices are used. In the case of the electricrefrigerator-freezer 5300, electric power can be stored in the powerstorage device 5304 at night time when the temperature is low and thedoor for a refrigerator 5302 and the door for a freezer 5303 are notopened or closed. The power storage device 5304 is used as an auxiliarypower supply in daytime when the temperature is high and the door for arefrigerator 5302 and the door for a freezer 5303 are opened and closed;thus, the usage rate of electric power in daytime can be reduced.

Further, the electric device illustrated in FIGS. 7A and 7B is anexample of a folding mobile information terminal. FIG. 7A is a schematicexternal view, and FIG. 7B is a block diagram.

The electric device in FIGS. 7A and 7B includes a housing 6000 a, ahousing 6000 b, a panel 6001 a, a panel 6001 b, a hinge 6002, a button6003, a connection terminal 6004, and a storage medium insertion portion6005, as illustrated in FIG. 7A. In addition, the electric device inFIGS. 7A and 7B has a power source portion 6101, a wirelesscommunication portion 6102, an arithmetic portion 6103, an audio portion6104, and a panel portion 6105, as illustrated in FIG. 7B.

The panel 6001 a is provided in the housing 6000 a.

The panel 6001 b is provided in the housing 6000 b. The housing 6000 bis connected to the housing 6000 a with the hinge 6002.

The panel 6001 a and the panel 6001 b function as display panels. Forexample, the panel 6001 a and the panel 6001 b may display differentimages or one image.

Further, one of or both the panel 6001 a and the panel 6001 b mayfunction as a touch panel. In this case, data may be input in such amanner that an image of a keyboard is displayed on one of or both thepanel 6001 a and the panel 6001 b and then touched with a finger 6010 orthe like. Alternatively, the display panel and the touch panel may bestacked, so that one of or both the panel 6001 a and the panel 6001 bare formed. Further alternatively, one of or both the panel 6001 a andthe panel 6001 b may be formed with use of an input-output panelprovided with a display circuit and a light detection circuit.

In the electric device illustrated in FIGS. 7A and 7B, the housing 6000a is overlapped with the housing 6000 b by moving the housing 6000 a orthe housing 6000 b with use of the hinge 6002, so that the electricdevice can be folded.

The button 6003 is provided on the housing 6000 b. Alternatively, thebutton 6003 may be provided on the housing 6000 a. Furtheralternatively, a plurality of buttons 6003 may be provided on one of orboth the housing 6000 a and the housing 6000 b. For example, when thebutton 6003 which is a power button is provided and pushed, the state ofthe electric device can be controlled, i.e., the electric device can beset to an on state or an off state, by putting the button 6003.

The connection terminal 6004 is provided on the housing 6000 a.Alternatively, the connection terminal 6004 may be provided on thehousing 6000 b. Further alternatively, a plurality of connectionterminals 6004 may be provided on one of or both the housing 6000 a andthe housing 6000 b. For example, when the electric device is connectedto a personal computer via the connection terminal 6004, data stored inthe electric device may be rewritten using the personal computer.

The storage medium insertion portion 6005 is provided on the housing6000 a. Alternatively, the storage medium insertion portion 6005 may beprovided on the housing 6000 b. Further alternatively, a plurality ofstorage medium insertion portions 6005 may be provided on one of or boththe housing 6000 a and the housing 6000 b. For example, when a cardstorage medium is inserted into the storage medium insertion portion,data can be read from the card storage medium and written into to theelectric device, or data can be read from the electric device andwritten to the card storage medium.

The power source portion 6101 has a function of supplying power fordriving the electric device. For example, from the power source portion6101, power is supplied to the wireless communication portion 6102, thearithmetic portion 6103, the audio portion 6104, and the panel portion6105. The power source portion 6101 is provided with a power storagedevice 6111. The power storage device 6111 is provided in one of or boththe housing 6000 a and the housing 6000 b. As the power storage device6111, the power storage device described in Embodiment 2 can beemployed. Note that a power supply circuit which generates a powersupply voltage for driving the electric device may be provided in thepower source portion 6101. In this case, in the power supply circuit,the power supply voltage is generated using power supplied from thepower storage device 6111. Further, the power source portion 6101 may beconnected to a commercial power supply.

The wireless communication portion 6102 has a function of transmittingand receiving electric waves. For example, the wireless communicationportion 6102 is provided with an antenna, a demodulation circuit, amodulation circuit, and the like. In this case, for example, electricwaves are transmitted and received at the antenna, whereby data isexchanged with an external device. Note that a plurality of antennas maybe provided in the wireless communication portion 6102.

The arithmetic portion 6103 has a function of conducting arithmeticprocessing in accordance with instruction signals input from thewireless communication portion 6102, the audio portion 6104, and thepanel portion 6105, for example. For example, the arithmetic portion6103 is provided with a CPU, a logic circuit, a memory circuit, and thelike.

The audio portion 6104 has a function of controlling input/output ofsound that is audio data. For example, the audio portion 6104 isprovided with a speaker and a microphone.

The power source portion 6101, the wireless communication portion 6102,the arithmetic portion 6103, and the audio portion 6104 are provided,for example, inside one of or both the housing 6000 a and the housing6000 b.

The panel portion 6105 has a function of controlling operation of thepanel 6001 a (also referred to as panel A) and the panel 6001 b (alsoreferred to as panel B). Note that a driver circuit, which controlsdriving of the panel 6001 a and the panel 6001 b, is provided in thepanel portion 6105, whereby operation of the panel 6001 a and the panel6001 b may be controlled.

Note that a control circuit may be provided in one or a plurality of thepower source portion 6101, the wireless communication portion 6102, thearithmetic portion 6103, the audio portion 6104, and the panel portion6105, thereby controlling operation. Further, a control circuit may beprovided in the arithmetic portion 6103, thereby controlling operationof one or a plurality of the power source portion 6101, the wirelesscommunication portion 6102, the audio portion 6104, and the panelportion 6105.

Further, a memory circuit may be provided in one or a plurality of thepower source portion 6101, the wireless communication portion 6102, theaudio portion 6104, and the panel portion 6105, whereby data necessaryfor operation may be stored in the memory circuit. Thus, operation speedcan be improved.

The electric device illustrated in FIGS. 7A and 7B can receive electricpower from the commercial power supply and use power stored in the powerstorage device 6111. Thus, even when electric power cannot be suppliedfrom the commercial power supply because of power outage or the like,the electric device can be operated with use of the power storage device6111 as a power supply.

When the structure shown in FIGS. 7A and 7B is employed, the electricdevice can have one or a plurality of functions of a telephone set, ane-book reader, a personal computer, and a game machine, for example.

The foregoing has described an example of an electric device accordingto this embodiment.

As described with reference to FIG. 6 and FIGS. 7A and 7B, in an exampleof an electric device according to this embodiment, a power storagedevice is provided, whereby the electric device can be driven withelectric power supplied from the power storage device. Thus, even when,for example, external electric power is not supplied, the electricdevice can be driven.

EXAMPLE 1

In this example, LiFePO₄ (lithium iron phosphate) formed by a productionmethod described in Embodiment 1 will be described.

First, a production method of LiFePO₄ is described.

In the production method of LiFePO₄ according to this example, rawmaterials were weighed so that a molar ratio,LiOH.H₂O:FeCl₂.4H₂O:NH₄H₂PO₄=2:1:1, was obtained. At this time, Fe wasweighed to have a concentration of 0.2 M with respect to 100 ml ofwater.

Next, in a nitrogen atmosphere, the weighed LiOH.H₂O, FeCl₂.4H₂O, andNH₄H₂PO₄ were each dissolved in 30 ml of water which had been subjectedto nitrogen bubbling, so that a solution containing Li, a solutioncontaining P, and a solution containing Fe were made.

Next, the solution containing Li was dripped into the solutioncontaining P in a nitrogen atmosphere, so that a solution containing Liand P was made. At this time, Li₃PO₄ was deposited in the mixedsolution.

Next, the solution containing Fe was dripped into the solutioncontaining Li and P in a nitrogen atmosphere, so that a mixture solutioncontaining Li, P, and Fe was made. At this time, a LiFePO₄ precursor wasdeposited in the mixture solution.

Next, 10 ml of water which had been subjected to nitrogen bubbling wasadded to the mixture solution. As a result, the amount of the mixturesolution was 100 ml. The oxygen concentration of the thus obtainedmixture solution was 4.5 ppm.

Next, after the mixture solution was transferred into an autoclave, thesolution was reacted for 15 hours at a temperature of 150° C. whilebeing stirred in a nitrogen atmosphere. Note that the pressure at thisstep was 0.4 MPa.

Then, the solution reacted in the autoclave was filtered in an airatmosphere, and the left compound was washed ten times with pure water.After washing, the compound was dried in a vacuum at a temperature of50° C.

Through the above steps, LiFePO₄ was formed.

In addition, the formed LiFePO₄ was observed with a scanning electronmicrograph (SEM). The observation result is shown in FIG. 8.

Furthermore, a comparison example LiFePO₄ was formed. In a process forforming the comparison example LiFePO₄, all the treatment was performedin an air atmosphere whereas some of the treatment in the process forforming the above-described LiFePO₄ was performed in a nitrogenatmosphere. Other than the atmosphere, the production conditions werethe same as those of the above LiFePO₄. The formed comparison exampleLiFePO₄ was observed with a SEM. The observation result is shown in FIG.9.

As shown in FIG. 8, the LiFePO₄ has a plurality of crystals which arerectangular solids. In addition, variation in crystal shape of theLiFePO₄ in FIG. 8 is small as compared with that of the LiFePO₄ in FIG.9. The above results indicate that variation in crystal shape in thecase where LiFePO₄ is formed in a nitrogen atmosphere can be reduced ascompared with the case where LiFePO₄ is formed in an air atmosphere.

EXPLANATION OF REFERENCE

161: rectangular solid, 201: positive electrode, 202: negativeelectrode, 203: electrolyte, 204: separator, 211: positive electrodecurrent collector, 212: positive electrode active material layer, 221:negative electrode current collector, 222: negative electrode activematerial layer, 301: positive electrode, 302: negative electrode, 304:separator, 305: housing, 306: housing, 307: ring-shaped insulator, 308:spacer, 309: washer, 5000: display device, 5001: housing, 5002: displayportion, 5003: speaker portion, 5004: power storage device, 5100:lighting device, 5101: housing, 5102: light source, 5103: power storagedevice, 5104: ceiling, 5105: wall, 5106: floor, 5107: window, 5200:indoor unit, 5201: housing, 5202: ventilation duct, 5203: power storagedevice, 5204: outdoor unit, 5300: electric refrigerator-freezer, 5301:housing, 5302: door for refrigerator, 5303: door for freezer, 5304:power storage device, 6000 a: housing, 6000 b: housing, 6001 a: panel,6001 b: panel, 6002: hinge, 6003: button, 6004: connection terminal,6005: storage medium insertion portion, 6010: finger, 6101: power sourceportion, 6102: wireless communication portion, 6103: arithmetic portion,6104: audio portion, 6105: panel portion, 6111: power storage device

This application is based on Japanese Patent Application serial no.2011-189027 filed with Japan Patent Office on Aug. 31, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A power storage device comprising: a positive electrode comprising a positive electrode current collector and a positive electrode active material layer on the positive electrode current collector, wherein the positive electrode active material layer comprises graphene and a plurality of crystals of composite oxide which is represented by a general formula LiMPO_(4,) wherein M denotes a transition metal selected from the group consisting of Fe, Co, Ni, and Mn, wherein the plurality of crystals is covered with the graphene, and wherein each of the plurality of crystals has a crystal shape of a rectangular solid.
 2. The power storage device according to claim 1, wherein each of the plurality of crystals has an olivine structure.
 3. The power storage device according to claim 1, wherein the graphene functions as a conductive auxiliary agent and a binder in the positive electrode active material layer.
 4. The power storage device according to claim 1, wherein proportion of an element other than carbon in the graphene is 30 at % or less.
 5. The power storage device according to claim 1, wherein proportion of an element other than carbon and hydrogen the graphene contains to account for 15 at % or less.
 6. The power storage device according to claim 1, further comprising a negative electrode facing the positive electrode, wherein the negative electrode comprises a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector.
 7. The power storage device according to claim 6, wherein the negative electrode active material layer comprises graphene.
 8. A display device comprising: the power storage device according to claim 1; a display portion electrically connected to the power storage device; and a speaker electrically connected to the power storage device.
 9. An electronic equipment comprising the power storage device according to claim 1, wherein the electronic equipment is configured to receive electric power stored in the power storage device.
 10. A power storage device comprising: a positive electrode comprising a positive electrode current collector and a positive electrode active material layer on the positive electrode current collector, wherein the positive electrode active material layer comprises graphene and a plurality of crystals of composite oxide which is represented by a general formula LiMPO_(4,) wherein M denotes a transition metal selected from the group consisting of Fe, Co, Ni, and Mn, wherein the graphene is multilayer graphene including 2 to 100 graphene sheets, wherein the plurality of crystals are provided between the graphene sheets, and wherein each of the plurality of crystals has a crystal shape of a rectangular solid.
 11. The power storage device according to claim 10, wherein each of the plurality of crystals has an olivine structure.
 12. The power storage device according to claim 10, wherein the graphene functions as a conductive auxiliary agent and a binder in the positive electrode active material layer.
 13. The power storage device according to claim 10, wherein proportion of an element other than carbon in the graphene is 30 at % or less.
 14. The power storage device according to claim 10, wherein proportion of an element other than carbon and hydrogen the graphene contains to account for 15 at. % or less.
 15. The power storage device according to claim 10, further comprising a negative electrode facing the positive electrode, wherein the negative electrode comprises a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector.
 16. The power storage device according to claim 15, wherein the negative electrode active material layer comprises graphene.
 17. A display device comprising: the power storage device according to claim 10; a display portion electrically connected to the power storage device; and a speaker electrically connected to the power storage device.
 18. An electronic equipment comprising the power storage device according to claim 10, wherein the electronic equipment is configured to receive electric power stored in the power storage device. 